Methane gas producing field is?

Methane is produced by methanogens, archaebacteria that work only under strictly anaerobic conditions. A waterlogged paddy field is the classic example. When the field is flooded, oxygen is driven out of the soil within days, and organic matter such as straw, stubble and roots decomposes anaerobically, releasing CH4 that escapes through the water and plant stems. So the answer is B.

Why the others are wrong:

• Wheat: an upland, well-drained crop grown without standing water, so the soil stays aerobic and methanogenesis is negligible.
• Cotton: also a dryland crop with no sustained anaerobic zone in the root area.
• Groundnut: a legume on well-drained soil. It fixes nitrogen but does not create flooded, oxygen-free conditions.

Core concept: anaerobic decomposition and biogenic methane, the same process used in biogas plants. Mains angle: rice cultivation is a major anthropogenic methane source in climate discussions, and Alternate Wetting and Drying (AWD) is promoted to cut emissions and water use, which fits GS-III on agriculture, climate change and water conservation.

Trap: candidates pick wheat or cotton because they are large-area crops, or confuse methane with the nitrogen fixation done by groundnut. The key word to hold on to is flooded, meaning anaerobic.